Ever more stringent legal regulations regarding the pollutant emissions of internal combustion engines demand that, for environmental protection reasons, the nitrogen oxides (NOx) emissions of internal combustion engines are continuously further reduced. Exhaust-gas aftertreatment with regard to nitrogen oxides is expensive, requires intensive servicing, may not be reliable and involves high maintenance costs. In particular, additional, complex components may be required, for example for a post-injection of fuel, carbamide (urea) or the like. It is sometimes necessary for the machine to be run in a separate operating range while the aftertreatment or regeneration processes take place.
The generation of nitrogen oxides is a direct result of the combustion process. This means that the generation of nitrogen oxides can also be influenced by a change to the combustion process, whereby the demand for an aftertreatment of the exhaust gas can be at least reduced. The generation of nitrogen oxides is promoted by high combustion temperatures and an abundance of oxygen in the combustion gas. It is accordingly sought to keep the combustion temperatures low and to dilute the combustion gas, that is to say, to lower the oxygen content in the combustion gas to the required minimum.
For this purpose, it is known for a part of the exhaust gas to be recirculated into the intake manifold of the internal combustion engine. This is referred to as exhaust-gas recirculation, EGR. An example of exhaust-gas recirculation can be found in DE 10 2009 046 370 A1.
Since the exhaust gas naturally has no oxygen or a small residual oxygen content, it constitutes an inert gas which is available continuously during operation. By adding EGR, the overall oxygen content in the gas mixture for the internal combustion engine can be reduced. The exhaust gas may either be branched off upstream of the exhaust-gas aftertreatment arrangement (for example a diesel particle filter) and supplied downstream of the compressor, or else may be branched off downstream of the exhaust-gas aftertreatment arrangement and supplied upstream of the compressor. The first case is also referred to as high-pressure exhaust-gas recirculation, because the exhaust gas is at high pressure during the recirculation, and the second case is, correspondingly, referred to as low-pressure exhaust-gas recirculation, because the exhaust gas experiences a pressure drop as it passes through the exhaust-gas aftertreatment arrangement and may subsequently be compressed again together with the inlet air by the compressor.
High-pressure exhaust-gas recirculation has the disadvantage that soot particles can accumulate in the intake manifold of the internal combustion engine, which impairs the scavenging efficiency, that is to say the purging of the exhaust gas out of the cylinder by the supply of the combustion gas for the subsequent ignition process. Low-pressure exhaust-gas recirculation substantially eliminates said disadvantage because exhaust gas which has already been purified by the exhaust-gas aftertreatment arrangement is recirculated. Here, however, the work performed by the compressor increases because more gas may be compressed; furthermore, the regulation of the low-pressure exhaust-gas recirculation is complex.
It is also known for water to be injected directly into the intake manifold in order to lower the combustion temperature. Such a device is described for example in DE 102 04 181 C1. Here, however, it is a disadvantage that a corresponding water reservoir is carried and regularly replenished, wherein the frequency of replenishment is determined by the water consumption.
In light of the above mentioned disadvantages, a first aspect of the disclosure therefore introduces a method for reduction of the nitrogen oxides emissions of an internal combustion engine. The method of the disclosure may be implemented with or without exhaust-gas recirculation, as a result of which the above-mentioned problems may be eliminated or minimized. The method of the present disclosure also minimizes the amount of water injected such that a correspondingly smaller water reservoir may be carried and the water reservoir replenished less frequently.
The present disclosure describes a system and method to reduced NOx emissions of an internal combustion engine by injecting water into an intake manifold, thereby reducing combustion chamber temperatures and diluting oxygen content of intake air. A method of the present disclosure may maintain a desired oxygen concentration as a function of the gas pressure and temperature in the intake manifold and the rate, or amount, of water injection into the intake manifold. Maintenance of the oxygen concentration ([O2]) is achieved, in part, by monitoring [O2] in the intake manifold and maintaining the relative humidity below 100% by further monitoring the temperature and pressure. Water injection is controlled responsive to a calculated relative humidity setpoint and desired oxygen concentration. Maintaining the relative humidity below 100% sustains the injected water in the vapor form. Furthermore, the object of the present disclosure may be used in conjunction with, or in the absence of exhaust-gas recirculation.
Systems and methods for reducing NOx emissions are provided, comprising: adjusting an amount of water injected into an intake manifold responsive to an oxygen concentration, temperature and pressure in the intake manifold; and heating the injected water if humidity is higher than a threshold. Water injected into the intake manifold decreases the temperature of, and dilutes the oxygen content of intake gases thereby decreasing NOx emissions.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. Further, the inventors herein have recognized the disadvantages noted herein, and do not admit them as known.